Sunan an-Nasa'i, Hadith 1623 ⓘ
It was narrated that Hudhaifah said: "We were commanded to use the siwak when we got up to pray at night."
أَخْبَرَنَا عَبْدُ اللَّهِ بْنُ سَعِيدٍ، عَنْ إِسْحَاقَ بْنِ سُلَيْمَانَ، عَنْ أَبِي سِنَانٍ، عَنْ أَبِي حُصَيْنٍ، عَنْ شَقِيقٍ، عَنْ حُذَيْفَةَ، قَالَ كُنَّا نُؤْمَرُ بِالسِّوَاكِ إِذَا قُمْنَا مِنَ اللَّيْلِ .
also referenced under: Book 20, Hadith 26 (In-Book) & Volume 2, Book 20, Hadith 1624 (DarusSalam)